create tests-only patches

[joachim-n]

Dorgpatch (this project's predecessor) is able to detect a secondary branch if it has a name of the form '12345-tests'. It's probably too complex to create this branch automatically from the list d.org patches.

But an alternative approach would be to do a git diff on only certain folders. We know where tests are going to be located, so this approach might work:

• list all files which have changed
• identify the ones which have tests. For contrib, I think this is /tests/ and /src/Tests. For core I'm not sure sure.
• output a patch of just the changes to those files.

[ryanaslett]

FYI: the d.o. testrunner plans on detecting the tests and applying them first to prove the tests fail initially : https://www.drupal.org/node/2569585

[joachim-n]

Cool! Thanks for letting me know. In that case, I'll hold off on this for now and see what happens with the testrunner.